In an electric vehicle provided with an electric motor such as a synchronous motor or the like serving as a generation source of a driving force of the vehicle, a driving torque output from the electric motor is transmitted to driving wheels of the vehicle via a power transmission mechanism such as a transmission or the like to generate the driving force for the vehicle.
When the electric vehicle of this type is on an ascending slope or the like, it is possible that an operation state of the vehicle is in a so-called stall state. The stall state refers to such a state that even though the driving torque generated in the electric motor according to manipulations of an accelerator or the like of the vehicle is being transmitted to the driving wheels of the vehicle, the vehicle is in a stopped state (a vehicular velocity is maintained at zero or almost at zero).
In the stall state, the electric motor generates the driving torque while a rotor thereof has stopped rotating.
In the electric motor of such electric vehicle, the armature windings thereof are generally connected to a direct current power source via an inverter circuit including a plurality of switching elements. The electric motor generates the driving torque by supplying the electric power to the armature windings from the direct current power source via the inverter circuit. At this moment, the on and off of the switching elements included in the inverter circuit is conducted in conjunction with the rotation of the rotor of the electric motor. Thus, in the above-mentioned stall state where the rotor of the electric motor has stopped rotating, a specific switching element in the inverter circuit is maintained at the ON state, and the current is kept flowing in the switching element. Thereby, it is liable for the switching element to become overheated.
Conventionally, as a vehicle control technique of the electric vehicle in the stall state, there have been known those disclosed in Patent documents 1, 2 and 3, for example.
According to the technique disclosed in Patent document 1, when the operation state of the vehicle falls into the stall state and the stall state has been lasted over an allowable time determined on the basis of a torque being generated by the electric motor, the torque of the electric motor is controlled to decrease so as to make a recession velocity or a recession acceleration of the vehicle equal to or lower than a predetermined value. Thereby, the switching element in the inverter circuit is prevented from becoming overheated.
Patent document 2 has disclosed a technique which makes the electric motor generate a torque so as to maintain the stopped state of the vehicle for a predetermined time after a brake operation (an operation on a brake pedal) is released when the electric vehicle is temporarily stopped in the travelling. According to the technique, after the predetermined time has elapsed until an accelerator operation (an operation on an accelerator pedal) is conducted to start up the vehicle, even though the brake operation is not conducted, a brake driving device is actuated automatically.
Patent document 3 has disclosed a technique which prevents the inverter circuit from becoming overheated by actuating the brake of the vehicle to immobilize the vehicle and meanwhile stops the electric motor from generating a torque if the temperature of the inverter circuit becomes equal to or greater than a predetermined temperature when the vehicle is in the stall state. According to the technique, as the temperature of the inverter circuit drops below the predetermined temperature, the brake is released and meanwhile the electric motor is made to generate a torque equal to or greater than a predetermined value.